How do I calculate the uplink and downlink carrier to noise ratio for a satellite link?
Satellite C/N
Intermodulation noise (C/I) arises when multiple carriers share a satellite transponder. The transponder's TWTA or SSPA operates near saturation for efficiency, creating intermodulation products between the carriers. The C/I depends on the number of carriers, the output back-off (OBO), and the transponder's AM-AM and AM-PM characteristics. Typical OBO for multi-carrier operation: 3-7 dB, resulting in C/I of 15-25 dB. Single-carrier operation can use lower OBO (higher efficiency).
Frequently Asked Questions
Why is the downlink usually the limiting link?
The satellite transmitter power is limited by the available DC power from solar panels (typically 5-20 kW for a GEO satellite, divided among many beams). The satellite transmitter EIRP is constrained, making the downlink the weaker link. Ground stations can more easily increase EIRP (larger antenna, higher-power amplifier) to ensure the uplink is strong.
How does ACM help?
Adaptive coding and modulation (ACM) adjusts the modulation order and forward error correction rate based on the real-time C/N. During clear sky: high modulation (16APSK, 32APSK) for maximum throughput. During rain fade: lower modulation (QPSK) with stronger FEC to maintain the link. ACM can provide 3-6 dB improvement in average throughput compared to fixed modulation.